It's not just gambling alone; what you said is true. When someone is excited, they can make huge promises to anyone. In this life, we should just learn not to put interest in the promises of someone if not we will get disappointed. What you can do is not say a word to him about the money, just let everything be and be his friend as usual.
If your friend had lost in his bet. You will notice he would stick to you for you to show compassion for him. Funny enough money won in gambling does not last forever, he will spend it all someday and then come back to you.
That's human for you, sometimes we can say things because of excitement and later act as if we never made any promises. There is no need to even talk about because the money belongs to him and he will make decision whether to give or not. I know this might look difficult for those who expect so much in cases like this when someone close offer to give a share out from their money, but if they have the understanding of how humans can easily change I believe it won't be a matter that needs to bother one. It is better to just move on just as if he never won the money because had it been he didn't win the game you their won't be any expectation from him.